Adjust facet heading style for small screens.

Closes #855 

#### Before
![before](https://cloud.githubusercontent.com/assets/96776/4225813/6f428104-3931-11e4-9008-23ee4be20694.png)

#### After
![after](https://cloud.githubusercontent.com/assets/96776/4225814/6f4484a4-3931-11e4-940d-1e296985570a.png)

![facet-heading](https://cloud.githubusercontent.com/assets/96776/4225818/770d7ba0-3931-11e4-9537-fb068fb9d5fa.gif)